Distribution and breeding sites of potential dengue vectors in Kandy and Nuwara Eliya districts of Sri Lanka

Entomological investigations were con­ ducted in 26 urban and semi urban areas in Kandy and Nuwara Eliya districts to determine the distribution and breeding sites of potential vectors of dengue, viz., Aedes aegypti Linnaeus and Aedes albopictus Skuse in these areas. Aedes larval surveys were carried out around suspected/ serologically positive dengue fever (DF)/ dengue haemorrhagic fever (DHF) cases in dengue transmission areas, and, in bus depots, government institutions and tyre shops (high risk places for Aedes breeding) in DF/ DHF non transmission areas. Aedes aegypti and Ae. albopictus were not encountered in the Nuwara Eliya town (altitude 1900 m). But, either Ae. albopictus or Ae. aegypti and Ae. albopictus were encountered in all the other localities (elevation 100 -1300 m). The majority (96%) of breeding sites of Ae. aegypti and Ae. albopictus comprised of water storage containers, discarded receptacles and tyres. Breeding sites of Ae. aegypti and Ae. albopictus differed from one locality to another as well as from one time period to another. This study shows that the areas situated below 1300 m elevation in Kandy and Nuwara Eliya districts are receptive for dengue transmission. Strengthening of DF/ DHF surveillance and control measures in these areas is an urgent need to reduce morbidity and mortality, and the spatial distribution of DF/DHF in the two districts. Since, the breeding sites differed from one period of time to another and from one locality to another, dengue control measures should be planned based on local and current entomological information.


Introduction
Aedes aegypti Linnaeus is important as the primary vector of dengue fever (DF) and dengue haemorrhagic fever (DHF) in tropical countries of the world.Aedes albopictus Skuse is considered as a secondary vector of DF in South East Asia and the Western Pacific (1).
In Sri Lanka, DF was first reported in early 1960s (2)

Results
Of the 26 study sites, neither Ae. aegypti nor Ae. albopictus was encountered in the Nuwara Eliya town.Ae. albopictus was encountered in Kalapitiya, Kurunduwatta, Hataraliyadda, Madugoda and Panwila.Both Ae. aegypti and Ae.albopictus were reported from the rest of the localities (Figl).

In a particular area, the types of breeding sites of Ae. aegypti and Ae. albopictus differ from one period of time to another (Table 2).
Water storage tanks are the most preferred breeding site of Ae. aegypti and Ae.albopictus while discarded receptacles and tyres serve as important breeding sites of the two vector mosquito species (Table 3).

Discussion
Since then, sporadic, progressively large and more frequent DF/DHF outbreaks occurred in the country.Over the past 10 years, the morbidity, mortality and the spatial distribution of the disease increased considerably with 15,434 suspected and serologically positive DF/DHF cases and 88 deaths for the year2004.At present, DF/DHF is endemic in many urban and semi urban areas, and has invaded new areas, making the disease an important public health problem in the country (3).Occurrence of seasonal/periodic peaks of DF/DHF in dengue endemic areas and emergence of new foci of transmission in hitherto DF/DHF transmission free areas are important problems faced by the dengue control programme in the country.Information on the distribution of Ae. aegypti and Ae.albopictus and their breeding sites in a particular area would definitely help to take timely and appropriate preventive and control measures to reduce the morbidity, mortality and the spatial distribution of the disease.This study was carried out to determine the distribution and breeding sites of Ae. aegypti and Ae.albopictus in Kandy and Nuwara Eliya districts of Sri Lanka.Methods Study area This study was carried out from January 2003 to December 2004 in the Kandy and Nuwara Eliya districts of Sri Lanka.The study area comprises of plain and hilly areas with a land area of 3,681 km 2 and an estimated mid year population of 2.04 million, for the year 2004 (Chief Secretariat, Planning and Monitoring Division, Kandy).According to the Regional Epidemiologists in the Kandy and Nuwara Eliya districts, the Kandy Municipality and the adjoining urbanized areas in the Divisional Director of Health Services (DDHS) areas, namely, Gangawatakorale, Harispattuwa, Kundasale, Pathadumbara, Udunuwara and Yatinuwara are endemic for DF/DHF.The Nuwara Eliya district has reported a few mini outbreaks during the past few years.Entomological investigations carried out in the area Twenty six urban and semi urban areas located at different altitudes in 24 DDHS areas in the Kandy and Nuwara Eliya districts were randomly selected for Aedes larval surveys.In DF/DHF transmission areas in the districts, Aedes larval surveys were carried out around the suspected and serologically positive DF/DHF cases that were notified mainly by the government hospitals.In hitherto DF/DHF non transmission areas, the surveys were carried out in and around bus depots, government institutions and tyre shops (high risk plaies for Aedes breeding) as these institutions reported frequent Ae. aegypti and Ae.albopictus breeding.During each survey, a minimum of 100 houses within a 200 m radius around the DF/DHF case or the high risk institution were visited, all possible Aedes breeding sites were examined and 20 larvae from each positive container were collected randomly by pipetting or dipping, depending on the nature of the breeding habitat.If a particular container had less than 20 larvae, all larvae were collected.The larvae were placed in separate containers (one container per one habitat), staged and identified as 3 rd and 4 th stages using standard keys (4, 5).The l sl and 2 nd stage larvae were allowed to develop to 3 rd and 4 th stages, and the pupae to become adults prior to identification.If, Ae. aegypti and/or Ae. albopictus were not encountered in a particular area at the first survey, two more surveys at more than 3 month intervals were carried out in the same area.Aedes mosquito density was determined by the Container Index (CI = percentage of containers positive for Ae.aegypti and Ae.albopictus) for Ae.aegypti and/or Ae, albopictus.Breeding sites of Ae. aegypti and Ae.albopictus were categorized into 5 groups, viz., (1) water storage containers (water storage cemented tanks and iron and plastic barrels), (2) discarded receptacles in the domestic and peridomestic environment (discarded plastic containers, pots, bottles, coconut shells, polythene, metal ware and tins), (3) tyres, (4) domestic appliances (refrigerator trays, flower vases and ant traps) and (5) others (eg., bamboo stump, leaf axils, concrete slabs and roof gutters).The relative importance of each category of containers for Ae.aegypti and Ae.albopictus breeding was determined based on the percentage positivity of each type of container for Ae.aegypti and Ae.albopictus breeding.